Theory If the encryption key is k≠0k\neq 0k=0, encrypt by z↦kzz\mapsto kzz↦kz. Decrypt by multiplying with the inverse k−1=1kk^{-1}=\dfrac{1}{k}k−1=k1. For k=a+bik=a+bik=a+bi: 1k=a−bia2+b2\frac{1}{k}=\frac{a-bi}{a^2+b^2}k1=a2+b2a−bi